A Short Course BookUsing Your Digital CameraA Guide To Great Photographs

Flash Sync and Shutter Speeds

Click to explore the flash sync speed.

Leaf shutters, common on point and shoot
cameras, allow faster flash sync speeds because they don't use curtains.

When you take a picture, the shutter opens and closes to let light strike the image sensor. When it does so, the shutter is fully open for a very short time. If the shutter speed is too fast, the burst of light from the flash won't fully expose all parts of the image sensor and part of the scene won't be captured in the image. The fastest shutter speed that can be used is called the flash synchronization speed and is usually between 1/125-1/500 second. If you select a faster shutter speed directly or indirectly, most cameras will override you and lower it. The shutter works with two curtains, a front and rear (sometimes called first and second curtains). The shutter opens when the front curtain slides out of the way and ends when the rear curtain slides to close it. The flash fires either when the shutter first fully opens or just before it's about to close.

Front/first curtain sync (the usual mode) means the flash fires when the
shutter's front curtain first fully opens to expose the image sensor.

Rear/second curtain sync means the flash fires just before the shutter's rear
curtain starts to close to end the exposure.

A focal plane shutter opens a curtain to begin an exposure and closes a second curtain to end it. At fast shutter speeds (top) the second curtain starts to end the exposure before the first curtain has fully opened so the two curtains form a slit traveling across the image sensor. Flash would only expose the area uncovered by the slit between the two rapidly moving curtains. At the flash sync speed and slower (bottom) the second curtain doesn't start to close until the first one is fully open.

Front curtain sync(top) fires the flash at the beginning of the exposure, then records ambient light. As a result, light streaks from the moving subject appear in front of it.

Rear curtain sync(bottom) fires the flash at the end of the exposure, after the ambient light has been recorded so the streaks trail behind the subject.